1. Field of the Invention
The present invention relates to lasers which provide an output beam having a high power and more particularly to unstable ring resonators.
2. Description of the Prior Art
Laser systems which provide an output beam at a very high power level such as those made possible with combustion driven chemical lasers are subject to various restraints which limit the actual output power from such a system. One of these limitations is due to a phenomenon which is commonly referred to as superfluorescence, a condition under which a gain medium having a population inversion produces spurious beams of laser radiation without an interaction with any reflecting surface. These spurious beams are undesirable because they deplete the population inversion which would otherwise be available for controlled stimulated emission from the working medium. Also these beams can be sufficiently powerful to expose various equipment in the area to damage.
One of the concepts advanced to increase the amount of working medium available without incurring superfluorescence effects is taught by Chenausky et al. in U.S. Pat. No. 3,921,096 entitled Unstable Split Mode Laser Resonator, filed on Dec. 16, 1974 and held with the present application by a common assignee. The essence of the split-mode resonator concept is the use of two separate volumes of gain medium, each of which has dimensions no greater tha the limitations imposed by superfluorescence. The individual volumes have no direct line of sight contact with one another although there is a region of common resonance in which the phase of the beam in each of the regions becomes locked to the phase of the beam from the other region. The invention is primarily for a high gain, short lifetime gaseous working medium which passes through the resonator in what is primarily an axial direction.
An unstable resonator concept having a radial mode pattern is disclosed by Chenausky et al in Application Ser. No. 568,083 entitled Unstable Laser Resonator Having Radial Propagation, filed on even date and held with the present application by a common assignee. The toroidal unstable resonator geometries described therein increase the volume of gain medium which can be accommodated without encountering superfluorescence which was not possible with the split-mode resonator previously described. The direction in which the gain medium flows is again essentially axial.
Another approach to the problem involves arranging the gain medium in the form of a cylindrical sheath such as that produced by radial flow configuration so that a relatively large volume can be handled without exceeding the superfluorescence length limitations. A relatively simple unstable resonator including an annular convex toroidal surface at one end of the cylindrical working medium and an annular concave toroidal mirror at the other end of the cylinder is feasible. The utility of such a resonator configuration is severely limited by the poor optical quality of the laser beam which results therefrom. A conventional unstable resonator arranged in such a cylindrical geometry has a very high Fresnel number which is defined as the square of the outer diameter of the cylindrical sheath of active medium divided by four times the product of the laser wavelength and the length of the cylinders. This high Fresnel number is an indication of very little coupling of the beam phase front around the circumference of the cavity as the laser radiation propagates between the annular end mirrors and through the circumferentially oriented gain medium. As a consequence such a resonator displays very poor mode discrimination and is prone to support high order azimuthal modes which have far field energy distributions exhibiting a minimum on axis and departing significantly from defraction limited operation.